The invention relates to a process for applying metallic sprayed coats by means of a thermal spraying process to the inner surface of a hollow body, the body being composed of a metal having a higher heat expansion coefficient than that of the coating material.
Considerable difficulties arise with regard to adhesion of the sprayed coat, which are caused by the variable heat expansion coefficient of the base material and the sprayed coat, especially when thicker sprayed coats (more than 0.3 mm) have to be applied by thermal spraying processes, such as plasma spraying, oxyacetylene or arc spraying processes. In addition to this, an increase of the separating destructive load in the adhesion surface occurs as a result of the inherent shrinkage of the coat which is applied in layers. The shrinkage tensions increase with the thickness of the sprayed coat and the coating speed, not only as a result of the summed up partial shrinkage of the sprayed layers, but also as a result of the increasing inherent strength of the coat, which thereby loads the undercoat with its own adhesion capacity and finally exceeds it.
An attempt has already been made to reduce the heat and shrinkage tensions occurring during spraying in hollow bodies by maintaining the hollow body at a certain temperature, i.e. to prevent an expansion of the hollow body which might lead to dislodging of the coat during coating. This measure does not, however, prevent a reduction of the adhesion of the sprayed coat in the case of coated hollow bodies which, in operation are exposed to very strong heating, such as, for example, cylinders of internal combustion engines.
It is the aim of the present invention to improve the adhesion of metallic sprayed coats which are applied thermally to the inner surfaces of hollow bodies.
According to the present invention there is provided a process for applying a metallic sprayed coat, by means of a thermal spraying process, to the inner surface of a hollow body which is composed of a metal having a heat expansion coefficient which is larger than that of the coating metal, in which the hollow body is heated to a temperature of over 150.degree. C. before coating and is cooled by at least 50.degree. C. during coating.
As a result of the process according to the invention, positive shrinkage tensions are created during the coating operation which continue to exist even during thermal loading of the hollow body and act to improve adhesion in operation.
Heat loss from the hollow body is preferably progressively increased in accordance with the increase in the thickness of the coat. This progressive cooling induces the existence of pressure shrinkage tensions during coating, which increases as the coat thickness increases, and cause the hollow body to embrace the sprayed coat with increasing force.
Cooling is advantageously carried out by evaporation of a fluid cooling medium, for example, water, on the outer surface of the wall of the hollow body.
The process according to the invention is primarily intended for spraying alloyed or non-alloyed steel or ferrotitanium onto the inner surfaces of hollow bodies of aluminum and aluminum alloys, but it can also be used, for example, for spraying carbide or oxide coats, for instance zircon oxide, onto the inner surfaces of steel hollow bodies.